The long term goal of this project is to understand early-onset familial Alzheimer's Disease (FAD) on a detailed molecular level by focusing on one family of proteins known to be mutated in the diseased state. Point mutations in presenilin-I (PS-I) have been shown to be responsible for an estimated 70% of the early-onset cases of FAD, with 38% of them located in exon 8 (the N-terminal portion of a145 amino acid loop). This implicates exon 8 as a possible hotspot for PS-I mutations and suggests it plays an important role in the brain of FAD patients. Understanding how these loop region mutations affect the functioning of the whole protein is critical to understanding how they lead to FAD. Our approach to understanding pathogenicity in this family of proteins is two-fold. First, a structural approach will be taken to study the wild type and selected mutant protein conformations in the TM6/7 loops region of PS-I and PS-II. Secondly, we will take clues from a physiologically-relevant binding partner, nicastrin, to decipher the functioning of wild type and mutant presenilins. Nicastrin has recently been shown to be part of the high molecular weigh complex that the presenilins form as they function in the proteolysis of the amyloid precursor protein (APP), leading to the formation of a 42 amino acid fragment, the Abeta peptide, which forms the plaques associated with Alzheimer's Disease. Understanding interactions between the members of this catalytically active high molecular weight complex is critical to designing new therapeutics.